The present invention relates generally to an image signal readout method for an image sensor and, more particularly, to an image signal readout method for an image sensor having a non-destructive readout characteristic.
Conventional CCD and MOS solid-state image sensors have what is called a destructive readout characteristic such that image information is extinguished by readout. In recent years, however, there has been developed a solid-state image sensor element having a hook structure which permits what is called non-destructive readout such that image information is not extinguished by readout. As described in detail, for instance, in Japanese Patent Applications Nos. 54001/80 and 60316/80, this image sensor element is equipped with a photodetecting function and a detected information storing function on account of such an arrangement that photocarriers are stored in a potential well of the hook structure. With the use of this image sensor element, it is not necessary to provide an image pickup section, a display section, such as a CRT, or a recording device for temporarily storing image information before recording image signals obtained on a magnetic tape or the like. Hence it is expected that this image sensor element will permit marked simplification of the construction of the image sensor and reduction of its cost as compared with the conventional image sensors.
In the studies for development of the image sensor capable of non-destructive readout, the present inventors encountered a new problem which had not been experienced with the conventional MOS and CCD types. That is, in the case where a number of such image sensor elements are connected to a common picture element signal output line and the picture elements to be read out are selected by scanning to sequentially read out their picture element information, the image information of each picture element is not destroyed even after the readout operation, so that the signal read out from the picture element remains on the common signal output line for a certain period of time, resulting in interference with the image signal read out from the next picture element. Of course, the signal output line has an electrostatic capacitance to ground of a certain magnitude and, in the state in which a switch between the picture element cell once read out and the signal output line is turned OFF to electrically isolate them, the image signal read out previously is attenuated with a time constant dependent on the product of the abovesaid capacitance value and the resistance value of the signal output line, so that the abovesaid problem is not so serious in an experimental device on a small scale in which the number of picture elements is small and the readout rate is low. For example, in the case of obtaining an image of picture quality equal to that of a still picture by silver salt photography (picture quality equal to an image enlarged six times using, for instance, a 110-size film) through utilization of this solid-state image sensor element, 512.times.767 picture elements, for instance, are needed. With such a large number of picture elements, the electrostatic capacitance to ground and the resistance value of the signal output line both increase to cause an increase in the discharge time and, on the other hand, the picture element readout intervals are reduced, resulting in the interference by the previous picture element rapidly increasing.